The insulation of electrically conductive wire used to form a coil or similar conductive article is generally established and may be undertaken by a number of methods, including coating the wire with an organic polymerized material. According to this approach, any one of several organic wire coatings selected from the group consisting of plastics, rubbers and elastomers will provide effective insulation on conductive material. Today most if not all electromagnetic coils use polymeric insulated wire.
However, while these materials demonstrate good dielectric properties and have the ability to withstand high voltages, they are compromised by their poor operating performance at temperatures above 220° C. as well as by their failure to effectively dissipate ohmic or resistance heating when used in coil windings. (Inorganic insulation such as glass, mica or certain ceramics, tolerates temperatures greater than 220° C. but suffer from being too brittle for most applications.)
In addition to coating conductive material with an organic substance, electrically conductive materials such as copper and aluminum may be anodized to provide some measure of insulation. In the case of a copper core, the anodization of this material is known to produce unsatisfactory results due to cracking. It is possible to electroplate copper with aluminum but this approach generally produces undesirable results in terms of durability of the coating. In the case of an aluminum core, copper can be plated on the core but results in unsatisfactory electrical efficiency.
An electrically insulated conductor for carrying signals or current having a solid or stranded copper core of various geometries with only a single electrically insulating and thermally conductive layer of anodized aluminum (aluminum oxide) is disclosed in U.S. Pat. No. 7,572,980. As described in the '980 patent, the device is made by forming uniform thickness thin sheet or foil of aluminum to envelop the copper conductive alloy core. The aluminum has its outer surface partially anodized either before or after forming to the core in an electrolytic process to form a single layer of aluminum oxide.
While the above-described developments represent advancements in the art of insulating wires, there remains room in the art for further advancement. For example, the known approaches are challenged by the oxide layer being scratched or cracked when wound on a spool to form the coil if the wire is fully anodized prior to the step of winding.